This invention relates to pipe fitting structures, and more particularly to a pipe coupling structure used for piping in chemical and other plants, oil and gas pipelines, well tubes, and like piping.
In the piping and pipelines referred to above, multiple-wall composite pipes are used for transporting fluids such as those containing corrosive substances. A typical multiple-wall composite pipe is a dual or double-wall pipe in which the inner wall is a liner tube made of a corrosion-resistant material for effective conducting of corrosive fluid and the outer wall or pipe is designed to provide strength to withstand internal pressures of the corrosive fluid and external forces. For example, there is a double-wall pipe comprising a stainless-steel inner pipe and a carbon-steel outer pipe in which the inner pipe is interference-fitted.
There are certain limitations to the length of such multiple-wall pipes due to restrictions in the manufacture of the pipes and to conditions in which the pipes are installed in site. Therefore, pipe couplings are used to obtain a desired length of piping as is well known in the art. As a matter of course, the pipe couplings are also required to have an internal corrosion resistance as well as a mechanical strength to withstand internal pressures and external forces.
A typical pipe coupling structure known heretofore comprises a tubular coupling member having on axially opposite parts thereof internal or female screw threads engaging outer or male screw threads provided on the end parts of the two pipes to be coupled, the coupling member having an intermediate annular ridge or projection directed radially inward thereof to be abuttingly interposed between the opposing end surfaces of the two pipes. The radially inner surface of the annular projection is made to have the same diameter as the inner diameter of the two pipes to be coupled whereby a smooth, continuous or uninterrupted cylindrical inner surface is formed from one pipe to the other. This means that the annular projection of the coupling member is exposed to the corrosive fluid or fluids flowing in the pipes.
As a consequence it has been customary to make the coupling member of a corrosion-resistant material such as stainless steel. This gives the coupling sufficient resistance to corrosive fluids but is disadvantageous in that the coupling is very expensive and lacks sufficient strength, thereby resulting in increased thickness and weight and in interference with other elements and devices in and along the pipe line.